You could install a CBR 954 rotor. its a direct bolt on to an 1800 rim (same offset as a 1800 rotor as well) and I believe its the same diameter as the 1300 rotor. I was going to try and bolt one on my bike but the fender mount on the 1800 interfered with the rotor but that shouldn't be a problem with a 1300.
If your interested I have a rotor I can sell you. Along with a 4 piston caliper for a cbr 954
I'll try and drum up a picture of my mock-up for ya.
2012 Victory Cross country